Phospholipids are a diverse class of amphipathic molecules present in all biological membranes. Because of the difficulties (only currently being resolved) encountered in working with membrane components, little is known about the enzymatic and genetic control of phospholipid metabolism, or about the role of individual phospholipid species in membrane function. A greater understanding of this aspect of membrane biochemistry would be desirable, since phospholipids are particularly abundant in structures susceptible to pathological processes, such as circulating lipoproteins and lung surfactant. Towards this end, a new approach has been developed for the rapid isolation and characterization of mutants defective in specific enzymes of phospholipid biosynthesis (Raetz, C.R.H., 1975, Proc. Nat. Acad. Sci. U.S.A. 72, 2274-2278). These mutants are altered in the synthesis of the "polar headgroups" of the membrane lipids, and have not been available previously (in distinction to glycerol and fatty acid auxotrophs which have been available for a long time). In the organisms to be studied here, it will be possible to vary the headgroup composition of the phospholipids normally found in the membrane and to introduce unusual phospholipids not ordinarily present. The growth properties, composition and membrane function of these mutants will be examined and compared to wild-type strains. Most of the studies described in this proposal will be carried out wth the simple bacterium, Escherichia coli, but the approach used here will be equally applicable to eukaryotic systems.